When satellites are launched into a geostationary orbit they typically go through a variety of stages prior to reaching a permanent station. This includes being launched into a transfer orbit before being "kicked" into the final geostationary orbit. To keep the satellite stable during the transfer (i.e. to keep the satellite from tumbling, rocking or wobbling) the vehicle is spin stabilized. This is done by on-board jets which spin the spacecraft to about 60 RPM about its flight axis.
The mechanical basis for spin stabilization is that an object spinning about one of the stable principle axes is relatively insensitive to small external disturbances. This is because the angular momentum vector is aligned with a principle axis and small disturbances will deviate the vector only slightly off axis. These are called stability points of a system.
To create this stable spin axis, the vehicle must be fairly well balanced. Any eccentricity could cause the spacecraft to wobble or even tumble. Knowledge of the center of gravity allows the satellite to be balanced for spin stabilization during orbital transfer.
The prior art shows many ways of determining center of gravity. For example, U.S. Pat. No. 4,726,222 issued to Schueller on Feb. 23, 1988, shows a fixture for determining the center of gravity of a torpedo which includes a cradle suspended pendulously in a support. The cradle has a pair of rollers grooved to receive the torpedo, and is provided with means for manually rotating the roller to displace the torpedo axially until the cradle becomes level in the support when the center of gravity may be observed or marked on the torpedo.
U.S. Pat. No. 3,388,589 issued to Campbell on June 18, 1968 discloses a center of gravity determining device which has a platform that is balanced in a horizontal position with a specimen thereon such that a vertical imaginary line passes through the center of gravity of the specimen and the platform so that marking means can be used to mark the upper and lower surfaces of the sample where the imaginary line passes through the specimen center of gravity.
U.S. Pat. No. 3,320,795 issued to Monroe et al on May 23, 1967 measures center of gravity by means of a device which utilizes counterweight on a balanced platform to re-balance the platform after the specimen has been placed on it.
U.S. Pat. No. 3,174,330 issued to Boundy on Mar. 23, 1965 provides apparatus which is used to locate the center of gravity of an object such as a missile in a manner which avoids the introduction of extraneous forces and effects other associated with the apparatus.
The hereinafter disclosed invention was required because of the need for finding the center of gravity of the so-called FLTSATCOM satellite. Motivation for the new method of determining the center of gravity of the satellite was derived from some of the special problems the module posed. One problem was that the module was not structurally stable enough to be tipped on end for balance measurements. The other problem was that the satellite package had an oscillator that had to be powered, and the power could be removed for only very short periods of time without disrupting the electronics. Therefore, some new way had to be found to measure the center of gravity, which was very accurate and, in addition, very easy and fast to operate. In accordance with this invention, static imbalance is used.
There are typically three methods of measuring the center of gravity of a volume statically. They include hanging the object from a wire, reaction measurement of the object and balance of the object on a pivot.
The hanging method is the simplest to perform when the object is small and accuracy is not important. When the subject of the experiment is hung from a wire, the center of gravity of the system lies along a line subtended by the wire. This means that if three measurements are made at three arbitrary locations, all three of the derived lines will intersect at the center of gravity.
The reaction measurement method relies on weighing the object with load cells and summing the moments about known locations to derive the center of gravity.
The balance method requires that the object be free to pivot about some known point. A restoring force is then applied to the object to level the pivot. By knowing the restoring force, the center of gravity may be calculated. The restoring force is typically a hanging weight or hydraulic piston.
None of the prior art known to the applicants was capable of meeting the requirements of applicant's system.